Numerous methods are known for the execution of electrophotography. At a general level, using a photoconductive material an electrostatic latent image is formed on an electrostatic image-bearing member (also referred to as a “photosensitive member” below) by various means. Then, a visible image is made by developing this latent image with toner; as necessary the toner image is transferred to a recording medium such as paper; and a copied article is obtained by fixing the toner image on the recording medium by, for example, the application of heat or pressure. For example, copiers and printers are image-forming apparatuses that use such an electrophotographic procedure.
These printers and copiers have in recent years been transitioning from analog to digital, and while there is strong demand for an excellent latent image reproducibility and high resolution, there is at the same time strong demand for size reduction, particularly with printers.
Previously, printers were connected in networks and such printers were often tasked with printing from a large number of people; however, in the last few years there has also been substantial demand for local printing in which the PC and printer are located on the individual's desktop. This has made it necessary to reduce the space taken up by printers and there is strong demand for printer downsizing.
Here, when printer downsizing is closely considered, it may be understood that mainly reducing the size of the fixing unit and reducing the size of the developing device (cartridge) will be effective for achieving size reduction. In particular, the latter accounts for a considerable portion of the volume of a printer, and it can thus be concluded that reducing the size of the developing device is essential for reducing printer size.
When the developing system is considered in this context, two-component development systems and monocomponent development systems are available as the printer development system; however, magnetic monocomponent development systems are best for size reduction. This is because they do not use components such as a carrier or a toner-coating roller.
Reducing the diameter of the electrostatic latent image-bearing member and the diameter of the toner-carrying member are effective when size reduction is considered in the case of magnetic monocomponent development systems; however, problems are also caused by reducing these diameters.
One of these problems is a phenomenon, known as “ghosting”, in which density irregularities appear in the image. A brief description of “ghosting” is provided in the following.
Development proceeds through the transfer of toner carried by the toner-carrying member to the electrostatic latent image. During this time, fresh toner is supplied to the regions where the toner on the surface of the toner-carrying member has been consumed (regions corresponding to image areas), while unconsumed toner remains present as such in regions where there has been no toner consumption (regions corresponding to nonimage areas). As a result, a difference in the amount of charging is produced between the freshly supplied toner (hereafter referred to as the supplied toner) and the toner that has remained present (hereafter referred to as the residual toner). Specifically, the freshly supplied toner has a relatively lower amount of charge and the toner that has remained present has a relatively higher amount of charge. Ghosting is produced due to this difference (refer to FIG. 1).
This difference in the amount of charging between the residual toner and the supplied toner is caused by the fact that the number of times the residual toner is subjected to charging grows to large values, in contrast to the fact that the supplied toner is subjected to charging, i.e., is passed through the contact region between the regulating blade and the toner-carrying member (referred to below as the contact region), a single time.
Furthermore, a small-diameter toner-carrying member means that the toner-carrying member will have a large curvature, resulting in a decline in the area of the contact region between the regulating blade and the toner-carrying member and a slow rise in toner charge. This causes a larger difference in the amount of charge between the supplied toner and the residual toner and a worsening of ghosting.
There have attempts to improve the preceding by controlling the flowability of the toner. These include, for example, adjusting the degree of agglomeration (Patent Document 1) and controlling the compression ratio of the toner (Patent Document 2). However, the effects are inadequate when the toner-carrying member has a small diameter due, as described above, to the small area of the contact region with the regulating blade. In addition, because the regulating blade generally has the opposite charging performance from the toner, the toner ends up sticking to the regulating blade and a uniform charge cannot be obtained. The improvement in ghosting is unsatisfactory as a result and additional improvements have been required.
On the other hand, in order to solve the problems associated with external additives, toners have been disclosed with a particular focus on the release of external additives (refer to, for example, Patent Documents 3 and 4). The charging performance of toners is again not adequately addressed in these cases.
Moreover, Patent Document 5 teaches stabilization of the development transfer steps by controlling the total coverage ratio of the toner base particles by the external additives, and a certain effect is in fact obtained by controlling the theoretical coverage ratio, provided by calculation, for a certain prescribed toner base particle. However, the actual state of binding by external additives is substantially different from the value calculated assuming the toner to be a sphere and such a theoretical coverage ratio does not correlate with the ghosting problem described above and improvement has been necessary.